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		<title>three.js webgl - gpgpu - flocking</title>
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			<a href="https://threejs.org" target="_blank" rel="noopener">three.js</a> - webgl gpgpu birds<br/>
			Move mouse to disturb birds.
		</div>

		<!--
		TODO: If you're reading this, you may wish to improve this example by
			- Create a better shading for the birds?

		-->

		<!-- shader for bird's position -->
		<script id="fragmentShaderPosition" type="x-shader/x-fragment">

			uniform float time;
			uniform float delta;

			void main()	{

				vec2 uv = gl_FragCoord.xy / resolution.xy;
				vec4 tmpPos = texture2D( texturePosition, uv );
				vec3 position = tmpPos.xyz;
				vec3 velocity = texture2D( textureVelocity, uv ).xyz;

				float phase = tmpPos.w;

				phase = mod( ( phase + delta +
					length( velocity.xz ) * delta * 3. +
					max( velocity.y, 0.0 ) * delta * 6. ), 62.83 );

				gl_FragColor = vec4( position + velocity * delta * 15. , phase );

			}

		</script>

		<!-- shader for bird's velocity -->
		<script id="fragmentShaderVelocity" type="x-shader/x-fragment">

			uniform float time;
			uniform float testing;
			uniform float delta; // about 0.016
			uniform float separationDistance; // 20
			uniform float alignmentDistance; // 40
			uniform float cohesionDistance; //
			uniform float freedomFactor;
			uniform vec3 predator;

			const float width = resolution.x;
			const float height = resolution.y;

			const float PI = 3.141592653589793;
			const float PI_2 = PI * 2.0;
			// const float VISION = PI * 0.55;

			float zoneRadius = 40.0;
			float zoneRadiusSquared = 1600.0;

			float separationThresh = 0.45;
			float alignmentThresh = 0.65;

			const float UPPER_BOUNDS = BOUNDS;
			const float LOWER_BOUNDS = -UPPER_BOUNDS;

			const float SPEED_LIMIT = 9.0;

			float rand( vec2 co ){
				return fract( sin( dot( co.xy, vec2(12.9898,78.233) ) ) * 43758.5453 );
			}

			void main() {

				zoneRadius = separationDistance + alignmentDistance + cohesionDistance;
				separationThresh = separationDistance / zoneRadius;
				alignmentThresh = ( separationDistance + alignmentDistance ) / zoneRadius;
				zoneRadiusSquared = zoneRadius * zoneRadius;


				vec2 uv = gl_FragCoord.xy / resolution.xy;
				vec3 birdPosition, birdVelocity;

				vec3 selfPosition = texture2D( texturePosition, uv ).xyz;
				vec3 selfVelocity = texture2D( textureVelocity, uv ).xyz;

				float dist;
				vec3 dir; // direction
				float distSquared;

				float separationSquared = separationDistance * separationDistance;
				float cohesionSquared = cohesionDistance * cohesionDistance;

				float f;
				float percent;

				vec3 velocity = selfVelocity;

				float limit = SPEED_LIMIT;

				dir = predator * UPPER_BOUNDS - selfPosition;
				dir.z = 0.;
				// dir.z *= 0.6;
				dist = length( dir );
				distSquared = dist * dist;

				float preyRadius = 150.0;
				float preyRadiusSq = preyRadius * preyRadius;


				// move birds away from predator
				if ( dist < preyRadius ) {

					f = ( distSquared / preyRadiusSq - 1.0 ) * delta * 100.;
					velocity += normalize( dir ) * f;
					limit += 5.0;
				}


				// if (testing == 0.0) {}
				// if ( rand( uv + time ) < freedomFactor ) {}


				// Attract flocks to the center
				vec3 central = vec3( 0., 0., 0. );
				dir = selfPosition - central;
				dist = length( dir );

				dir.y *= 2.5;
				velocity -= normalize( dir ) * delta * 5.;

				for ( float y = 0.0; y < height; y++ ) {
					for ( float x = 0.0; x < width; x++ ) {

						vec2 ref = vec2( x + 0.5, y + 0.5 ) / resolution.xy;
						birdPosition = texture2D( texturePosition, ref ).xyz;

						dir = birdPosition - selfPosition;
						dist = length( dir );

						if ( dist < 0.0001 ) continue;

						distSquared = dist * dist;

						if ( distSquared > zoneRadiusSquared ) continue;

						percent = distSquared / zoneRadiusSquared;

						if ( percent < separationThresh ) { // low

							// Separation - Move apart for comfort
							f = ( separationThresh / percent - 1.0 ) * delta;
							velocity -= normalize( dir ) * f;

						} else if ( percent < alignmentThresh ) { // high

							// Alignment - fly the same direction
							float threshDelta = alignmentThresh - separationThresh;
							float adjustedPercent = ( percent - separationThresh ) / threshDelta;

							birdVelocity = texture2D( textureVelocity, ref ).xyz;

							f = ( 0.5 - cos( adjustedPercent * PI_2 ) * 0.5 + 0.5 ) * delta;
							velocity += normalize( birdVelocity ) * f;

						} else {

							// Attraction / Cohesion - move closer
							float threshDelta = 1.0 - alignmentThresh;
							float adjustedPercent;
							if( threshDelta == 0. ) adjustedPercent = 1.;
							else adjustedPercent = ( percent - alignmentThresh ) / threshDelta;

							f = ( 0.5 - ( cos( adjustedPercent * PI_2 ) * -0.5 + 0.5 ) ) * delta;

							velocity += normalize( dir ) * f;

						}

					}

				}



				// this make tends to fly around than down or up
				// if (velocity.y > 0.) velocity.y *= (1. - 0.2 * delta);

				// Speed Limits
				if ( length( velocity ) > limit ) {
					velocity = normalize( velocity ) * limit;
				}

				gl_FragColor = vec4( velocity, 1.0 );

			}

		</script>

		<script type="x-shader/x-vertex" id="birdVS">

			attribute vec2 reference;
			attribute float birdVertex;

			attribute vec3 birdColor;

			uniform sampler2D texturePosition;
			uniform sampler2D textureVelocity;

			varying vec4 vColor;
			varying float z;

			uniform float time;

			void main() {

				vec4 tmpPos = texture2D( texturePosition, reference );
				vec3 pos = tmpPos.xyz;
				vec3 velocity = normalize(texture2D( textureVelocity, reference ).xyz);

				vec3 newPosition = position;

				if ( birdVertex == 4.0 || birdVertex == 7.0 ) {
					// flap wings
					newPosition.y = sin( tmpPos.w ) * 5.;
				}

				newPosition = mat3( modelMatrix ) * newPosition;


				velocity.z *= -1.;
				float xz = length( velocity.xz );
				float xyz = 1.;
				float x = sqrt( 1. - velocity.y * velocity.y );

				float cosry = velocity.x / xz;
				float sinry = velocity.z / xz;

				float cosrz = x / xyz;
				float sinrz = velocity.y / xyz;

				mat3 maty =  mat3(
					cosry, 0, -sinry,
					0    , 1, 0     ,
					sinry, 0, cosry

				);

				mat3 matz =  mat3(
					cosrz , sinrz, 0,
					-sinrz, cosrz, 0,
					0     , 0    , 1
				);

				newPosition =  maty * matz * newPosition;
				newPosition += pos;

				z = newPosition.z;

				vColor = vec4( birdColor, 1.0 );
				gl_Position = projectionMatrix *  viewMatrix  * vec4( newPosition, 1.0 );
			}

		</script>

		<!-- bird geometry shader -->
		<script type="x-shader/x-fragment" id="birdFS">

			varying vec4 vColor;
			varying float z;

			uniform vec3 color;

			void main() {
				// Fake colors for now
				float z2 = 0.2 + ( 1000. - z ) / 1000. * vColor.x;
				gl_FragColor = vec4( z2, z2, z2, 1. );

			}

		</script>

		<script type="importmap">
			{
				"imports": {
					"three": "../build/three.module.js",
					"three/addons/": "./jsm/"
				}
			}
		</script>

		<script type="module">

			import * as THREE from 'three';

			import Stats from 'three/addons/libs/stats.module.js';
			import { GUI } from 'three/addons/libs/lil-gui.module.min.js';

			import { GPUComputationRenderer } from 'three/addons/misc/GPUComputationRenderer.js';

			/* TEXTURE WIDTH FOR SIMULATION */
			const WIDTH = 32;

			const BIRDS = WIDTH * WIDTH;

			// Custom Geometry - using 3 triangles each. No UVs, no normals currently.
			class BirdGeometry extends THREE.BufferGeometry {

				constructor() {

					super();

					const trianglesPerBird = 3;
					const triangles = BIRDS * trianglesPerBird;
					const points = triangles * 3;

					const vertices = new THREE.BufferAttribute( new Float32Array( points * 3 ), 3 );
					const birdColors = new THREE.BufferAttribute( new Float32Array( points * 3 ), 3 );
					const references = new THREE.BufferAttribute( new Float32Array( points * 2 ), 2 );
					const birdVertex = new THREE.BufferAttribute( new Float32Array( points ), 1 );

					this.setAttribute( 'position', vertices );
					this.setAttribute( 'birdColor', birdColors );
					this.setAttribute( 'reference', references );
					this.setAttribute( 'birdVertex', birdVertex );

					// this.setAttribute( 'normal', new Float32Array( points * 3 ), 3 );


					let v = 0;

					function verts_push() {

						for ( let i = 0; i < arguments.length; i ++ ) {

							vertices.array[ v ++ ] = arguments[ i ];

						}

					}

					const wingsSpan = 20;

					for ( let f = 0; f < BIRDS; f ++ ) {

						// Body

						verts_push(
							0, - 0, - 20,
							0, 4, - 20,
							0, 0, 30
						);

						// Wings

						verts_push(
							0, 0, - 15,
							- wingsSpan, 0, 0,
							0, 0, 15
						);

						verts_push(
							0, 0, 15,
							wingsSpan, 0, 0,
							0, 0, - 15
						);

					}

					for ( let v = 0; v < triangles * 3; v ++ ) {

						const triangleIndex = ~ ~ ( v / 3 );
						const birdIndex = ~ ~ ( triangleIndex / trianglesPerBird );
						const x = ( birdIndex % WIDTH ) / WIDTH;
						const y = ~ ~ ( birdIndex / WIDTH ) / WIDTH;

						const c = new THREE.Color(
							0x666666 +
							~ ~ ( v / 9 ) / BIRDS * 0x666666
						);

						birdColors.array[ v * 3 + 0 ] = c.r;
						birdColors.array[ v * 3 + 1 ] = c.g;
						birdColors.array[ v * 3 + 2 ] = c.b;

						references.array[ v * 2 ] = x;
						references.array[ v * 2 + 1 ] = y;

						birdVertex.array[ v ] = v % 9;

					}

					this.scale( 0.2, 0.2, 0.2 );

				}

			}

			//

			let container, stats;
			let camera, scene, renderer;
			let mouseX = 0, mouseY = 0;

			let windowHalfX = window.innerWidth / 2;
			let windowHalfY = window.innerHeight / 2;

			const BOUNDS = 800, BOUNDS_HALF = BOUNDS / 2;

			let last = performance.now();

			let gpuCompute;
			let velocityVariable;
			let positionVariable;
			let positionUniforms;
			let velocityUniforms;
			let birdUniforms;

			init();

			function init() {

				container = document.createElement( 'div' );
				document.body.appendChild( container );

				camera = new THREE.PerspectiveCamera( 75, window.innerWidth / window.innerHeight, 1, 3000 );
				camera.position.z = 350;

				scene = new THREE.Scene();
				scene.background = new THREE.Color( 0xffffff );
				scene.fog = new THREE.Fog( 0xffffff, 100, 1000 );

				renderer = new THREE.WebGLRenderer();
				renderer.setPixelRatio( window.devicePixelRatio );
				renderer.setSize( window.innerWidth, window.innerHeight );
				renderer.setAnimationLoop( animate );
				container.appendChild( renderer.domElement );

				initComputeRenderer();

				stats = new Stats();
				container.appendChild( stats.dom );

				container.style.touchAction = 'none';
				container.addEventListener( 'pointermove', onPointerMove );

				//

				window.addEventListener( 'resize', onWindowResize );

				const gui = new GUI();


				const effectController = {
					separation: 20.0,
					alignment: 20.0,
					cohesion: 20.0,
					freedom: 0.75
				};

				const valuesChanger = function () {

					velocityUniforms[ 'separationDistance' ].value = effectController.separation;
					velocityUniforms[ 'alignmentDistance' ].value = effectController.alignment;
					velocityUniforms[ 'cohesionDistance' ].value = effectController.cohesion;
					velocityUniforms[ 'freedomFactor' ].value = effectController.freedom;

				};

				valuesChanger();

				gui.add( effectController, 'separation', 0.0, 100.0, 1.0 ).onChange( valuesChanger );
				gui.add( effectController, 'alignment', 0.0, 100, 0.001 ).onChange( valuesChanger );
				gui.add( effectController, 'cohesion', 0.0, 100, 0.025 ).onChange( valuesChanger );
				gui.close();

				initBirds();

			}

			function initComputeRenderer() {

				gpuCompute = new GPUComputationRenderer( WIDTH, WIDTH, renderer );

				const dtPosition = gpuCompute.createTexture();
				const dtVelocity = gpuCompute.createTexture();
				fillPositionTexture( dtPosition );
				fillVelocityTexture( dtVelocity );

				velocityVariable = gpuCompute.addVariable( 'textureVelocity', document.getElementById( 'fragmentShaderVelocity' ).textContent, dtVelocity );
				positionVariable = gpuCompute.addVariable( 'texturePosition', document.getElementById( 'fragmentShaderPosition' ).textContent, dtPosition );

				gpuCompute.setVariableDependencies( velocityVariable, [ positionVariable, velocityVariable ] );
				gpuCompute.setVariableDependencies( positionVariable, [ positionVariable, velocityVariable ] );

				positionUniforms = positionVariable.material.uniforms;
				velocityUniforms = velocityVariable.material.uniforms;

				positionUniforms[ 'time' ] = { value: 0.0 };
				positionUniforms[ 'delta' ] = { value: 0.0 };
				velocityUniforms[ 'time' ] = { value: 1.0 };
				velocityUniforms[ 'delta' ] = { value: 0.0 };
				velocityUniforms[ 'testing' ] = { value: 1.0 };
				velocityUniforms[ 'separationDistance' ] = { value: 1.0 };
				velocityUniforms[ 'alignmentDistance' ] = { value: 1.0 };
				velocityUniforms[ 'cohesionDistance' ] = { value: 1.0 };
				velocityUniforms[ 'freedomFactor' ] = { value: 1.0 };
				velocityUniforms[ 'predator' ] = { value: new THREE.Vector3() };
				velocityVariable.material.defines.BOUNDS = BOUNDS.toFixed( 2 );

				velocityVariable.wrapS = THREE.RepeatWrapping;
				velocityVariable.wrapT = THREE.RepeatWrapping;
				positionVariable.wrapS = THREE.RepeatWrapping;
				positionVariable.wrapT = THREE.RepeatWrapping;

				const error = gpuCompute.init();

				if ( error !== null ) {

					console.error( error );

				}

			}

			function initBirds() {

				const geometry = new BirdGeometry();

				// For Vertex and Fragment
				birdUniforms = {
					'color': { value: new THREE.Color( 0xff2200 ) },
					'texturePosition': { value: null },
					'textureVelocity': { value: null },
					'time': { value: 1.0 },
					'delta': { value: 0.0 }
				};

				// THREE.ShaderMaterial
				const material = new THREE.ShaderMaterial( {
					uniforms: birdUniforms,
					vertexShader: document.getElementById( 'birdVS' ).textContent,
					fragmentShader: document.getElementById( 'birdFS' ).textContent,
					side: THREE.DoubleSide

				} );

				const birdMesh = new THREE.Mesh( geometry, material );
				birdMesh.rotation.y = Math.PI / 2;
				birdMesh.matrixAutoUpdate = false;
				birdMesh.updateMatrix();

				scene.add( birdMesh );

			}

			function fillPositionTexture( texture ) {

				const theArray = texture.image.data;

				for ( let k = 0, kl = theArray.length; k < kl; k += 4 ) {

					const x = Math.random() * BOUNDS - BOUNDS_HALF;
					const y = Math.random() * BOUNDS - BOUNDS_HALF;
					const z = Math.random() * BOUNDS - BOUNDS_HALF;

					theArray[ k + 0 ] = x;
					theArray[ k + 1 ] = y;
					theArray[ k + 2 ] = z;
					theArray[ k + 3 ] = 1;

				}

			}

			function fillVelocityTexture( texture ) {

				const theArray = texture.image.data;

				for ( let k = 0, kl = theArray.length; k < kl; k += 4 ) {

					const x = Math.random() - 0.5;
					const y = Math.random() - 0.5;
					const z = Math.random() - 0.5;

					theArray[ k + 0 ] = x * 10;
					theArray[ k + 1 ] = y * 10;
					theArray[ k + 2 ] = z * 10;
					theArray[ k + 3 ] = 1;

				}

			}

			function onWindowResize() {

				windowHalfX = window.innerWidth / 2;
				windowHalfY = window.innerHeight / 2;

				camera.aspect = window.innerWidth / window.innerHeight;
				camera.updateProjectionMatrix();

				renderer.setSize( window.innerWidth, window.innerHeight );

			}

			function onPointerMove( event ) {

				if ( event.isPrimary === false ) return;

				mouseX = event.clientX - windowHalfX;
				mouseY = event.clientY - windowHalfY;

			}

			//

			function animate() {

				render();
				stats.update();

			}

			function render() {

				const now = performance.now();
				let delta = ( now - last ) / 1000;

				if ( delta > 1 ) delta = 1; // safety cap on large deltas
				last = now;

				positionUniforms[ 'time' ].value = now;
				positionUniforms[ 'delta' ].value = delta;
				velocityUniforms[ 'time' ].value = now;
				velocityUniforms[ 'delta' ].value = delta;
				birdUniforms[ 'time' ].value = now;
				birdUniforms[ 'delta' ].value = delta;

				velocityUniforms[ 'predator' ].value.set( 0.5 * mouseX / windowHalfX, - 0.5 * mouseY / windowHalfY, 0 );

				mouseX = 10000;
				mouseY = 10000;

				gpuCompute.compute();

				birdUniforms[ 'texturePosition' ].value = gpuCompute.getCurrentRenderTarget( positionVariable ).texture;
				birdUniforms[ 'textureVelocity' ].value = gpuCompute.getCurrentRenderTarget( velocityVariable ).texture;

				renderer.render( scene, camera );

			}

		</script>
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